Predictive video signal coding and decoding with suppression of low frequencies

ABSTRACT

In a predictive coding apparatus, an interframe/interfield predictive signal is generated in which low frequency components are depressed. A predictive error signal is generated on the basis of an input video signal and the interframe/interfield predictive signal. The predictive error signal is then interframe/interfield-coded. In a predictive decoding apparatus, a code transmitted from the coding apparatus is interframe/interfield-decoded to obtain a decoded signal. An interframe/interfield predictive signal is generated in which low frequency components are depressed. The video signal is then reproduced by adding the decoded signal and the predictive signal.

This is a divisional of application Ser. No. 08/731,124 filed Oct. 9,1996 which in turn is a divisional of application Ser. No. 08/286,205filed Aug. 5, 1994, now abandoned, which is a continuation of Ser. No.07/967,869, filed Oct. 29, 1992, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus that encodes video signalsefficiently with fewer codes and an apparatus that decodes the codedsignals, in their use for recording, transmission and display systems,and more particularly to a coding/decoding apparatus for video signalscapable of random access and search.

In particular, efficient coding has been realized by interframepredictive coding which utilizes interframe correlation of video signalsto predict, from already coded frames, the frames to be coded, andencodes predictive error signals alone.

Furthermore, recent attention has been directed to the technology ofmotion-compensating interframe prediction to predict motion by movingpictures.

On the other hand, it has been made for the coding for storage media torespond to random access and high-speed search.

A conventional coding apparatus is exemplified in FIG. 1.

In FIG. 1, a video signal applied to a video signal input terminal 1 isled to a predictive subtracter 3, where a predictive signal given via aswitch 15 is subtracted from the video signal, thus generating apredictive error signal.

The predictive error signal is encoded by an intraframe encoder 5 intocompressed data. The data is outputted via a data output terminal 7 andapplied to an intraframe decoder 9 which generates a reproducedpredictive error signal.

In an inverse predictive adder 11, a predictive signal is added to thereproduced predictive error signal, thus reproducing the video signal.

The reproduced video signal is stored in a frame memory (FM) 13, andapplied to a terminal P of the switch 15 after being delayed by oneframe.

The switch 15 is controlled by a synchronous signal separated from theinputted video signal. And, in an independent frame of every N frames, aterminal S is connected to a terminal I, whereas in other predictiveframes the terminal S is connected to terminal P.

The terminal P has been supplied with a reproduced video signal of thepreceding frame. An interframe predictive coding circuit is formed whenthe terminal S is connected to the terminal P.

The terminal I has been supplied with a fixed value (0). When theterminal S is connected to the terminal I, the output signal of thepredictive subtracter 3 becomes the same as the input video signal, thatis, an intraframe independent coding circuit is formed.

FIG. 2 shows a conventional video signal decoding apparatus. In thisapparatus, the data supplied from a data input terminal 17 is decoded byan intraframe decoder 19.

And in a reverse predictive adder 21, a predictive signal applied via aswitch 27 is added to the decoded signal, thus reproducing the videosignal.

The reproduced video signal is outputted via a video signal outputterminal 23, and is concurrently stored in a frame memory 25.

The signal is delayed by one frame in the frame memory 25, and thenapplied to the reverse predictive adder 21 via the switch 27. The switch27 is controlled by a frame-synchronizing signal separated from theinput data as in the case of the coding apparatus.

In the intraframe coding apparatus 5 of FIG. 1a, discrete cosinetransform (DCT) is first carried out. The transformed output is thenquantized, and the quantized data is variable-length encoded by suchcodes as Huffman codes.

In the intraframe decoders 9 and 19 of FIGS. 1 and 2, the data coded tovariable lengths is first decoded to a fixed length and then transformedto a quantized representative value, which is reversely discrete cosinetransformed into a reproduced signal.

Such conventional coding/decoding apparatuses process periodicindependent frames. This decreases the coding efficiency because theindependent frames produce a large amount of data as compared with thepredictive frames.

Independent frames only are reproduced in the high-speed search instorage media. However, if the quantity of data that can be decoded bythe high-speed search is the same as that at the time of ordinaryregeneration, not all of the independent frames corresponding to thesearch speed can be decoded and the decoding apparatus comes to deliverthe same frames repeatedly. This is so because the independent framescontain a larger amount of data.

Therefore, it has been devised to divide the frequency components of avideo signal into a plurality of bands and decode low frequencycomponents only.

Such a measure can reduce the amount of data to be decoded in each frameand provide smooth search pictures even in low resolution, but leaveshigh frequency components unused.

SUMMARY OF THE INVENTION

In view of the foregoing, it is a purpose of the present invention toprovide video signal predictive coding/decoding apparatuses in whichintraframe coding is carried out on low frequency components on whichthe energy of video signals is concentrated and intraframe predictivecoding is always carried out on high frequency components.

To achieve the foregoing purpose, the present invention provides anapparatus for coding a video signal.

The apparatus comprises, a first generator for generating aninterframe/field predictive signal in which low frequency components aredepressed, a second generator for generating a predictive error signalon the basis of an input video signal and the interframe/fieldpredictive signal, and an encoder for intraframe/field-coding thepredictive error signal.

Furthermore, the present invention provides an apparatus for decoding avideo signal. The apparatus comprises a decoder forintraframe/field-decoding a code transmitted from a coding apparatus toobtain a decoded signal, a generator for generating an interframe/fieldpredictive signal in which low frequency components are depressed, and areproducer for reproducing the video signal by adding the decoded errorsignal and the predictive signal.

Still furthermore, the present invention provides an apparatus forcoding a video signal. The apparatus comprises a separator forseparating an input video signal to be coded into high and low frequencycomponent signals, an encoder for generating an interframe/fieldpredictive error signal on the basis of the high frequency componentsignal, and a predictive encoder for interframe/field-predictive codingthe low frequency component signal and the predictive error signal.

Still furthermore, the present invention provides an apparatus fordecoding a video signal. The apparatus comprises a first decoder forintraframe/field-decoding codes of high and low frequency componentsignals, respectively, which codes have been transmitted from a codingapparatus, thereby to obtain decoded high and low frequency componentsignals, a first reproducer for adding the decoded high frequencycomponent signal and an interframe/field predictive signal, and a secondreproducer for adding the added signal from the above adding means andthe decoded low frequency component signal, thereby to obtain areproduced signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a conventional predictive codingapparatus;

FIG. 2 is a block diagram showing a conventional predictive decodingapparatus;

FIG. 3 is a block diagram of an embodiment of a predictive codingapparatus of the present invention;

FIG. 4 illustrates a configuration of a low pass filter (LPF) used forthe present invention;

FIGS. 5(a), 5(b), 5(c) and 5(d) show frequency characteristics ofsignals processed by the coding apparatus shown in FIG. 3;

FIG. 6 is a block diagram illustrating an embodiment of a predictivedecoding apparatus corresponding to the coding apparatus shown in FIG.3;

FIG. 7 is a block diagram of another embodiment of a predictive codingapparatus of the present invention;

FIG. 8 is a block diagram representing an embodiment of a predictivedecoding apparatus corresponding to the coding apparatus shown in FIG.7;

FIG. 9 is a block diagram illustrating still another embodiment of apredictive coding apparatus of the present invention;

FIG. 10 is a block diagram illustrating an embodiment of a predictivedecoding apparatus corresponding to the coding apparatus of FIG. 9; and

FIGS. 11A to 11D portray the frame structure in respective embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a block diagram illustrating an embodiment of the predictivecoding apparatus of the present invention.

In FIG. 3 a video signal applied to a picture signal input terminal 29is supplied to a predictive subtractor 31. The predictive subtractor 31subtracts a predictive signal supplied by a subtractor 33 from the inputvideo signal, and sends a generated predictive error signal to anintratrame encoder 35.

The encoder 35 encodes the predictive error signal by intratrame codingand delivers coded data from a data output terminal 37 and also sendsthe coded data to an intraframe decoder 39.

The predictive signal in the coding apparatus must be composed of anintraframe coded signal in order to be the same signal as a predictivesignal in a decoding apparatus described later. Therefore, theintraframe decoder 39 decodes the coded data to generate a reproducedpredictive error signal which is then applied to a reverse predictiveadder 41.

The adder 41 adds the reproduced predictive error signal and itscorresponding predictive signal to generate a reproduced video signalwhich is then applied to a frame memory (FM) 43.

The frame memory 43 provides a signal delayed by one frame from theinput video signal, and sends the delayed signal to a low pass filter(LPF) 45 and the subtractor 33. The output of the LPF 45 is supplied toa terminal I of a switch 47.

The switch 47 is controlled by a synchronous signal separated from theinput video signal. In one independent frame of every N frames aterminal S is connected to the terminal I, and in the other frames theterminal S is connected to a terminal P.

The terminal I is supplied with a low frequency component of areproduced video signal in the frame precedent to the input videosignal, whereas terminal P is maintained at a fixed level (0).

When the terminal S of the switch 47 is connected to the terminal P, thesubtractor 33 sends out an output signal of the frame memory 43, as itis, and therefore the predictive subtractor 31 makes predictions in thesame way as in conventional ones.

On the other hand, when the terminal S of the switch 47 is connected tothe terminal I, a low frequency component from LPF 45 is subtracted fromthe output signal of the frame memory 43, and the resulting predictivesignal is delivered from the subtracter 33.

Therefore, when in the predictive subtractor 31 the predictive signal issubtracted from the input video signal, the low frequency componentremains and is intraframe-coded in the intraframe encoder 35.

The LPF 45 is a two-dimensional filter using a cascade connection ofvertical and horizontal filters as shown in FIG. 4. The numerals in FIG.4 designate tap factors.

FIGS. 5(a)-5(b) illustrates frequency characteristics of respectivesignals processed by the predictive coding apparatus of FIG. 3.

The input video signal shown in FIG. 5(a) has all frequency componentsof the high and low frequencies. In the predictive signal shown in FIG.5(b) the low frequency component is suppressed.

In the predictive error signal, shown in FIG. 5(c), obtained bysubtracting such a predictive signal from the video signal shown in FIG.5(a), the input video signal remains unchanged as to the low frequencycomponent, and the predictive error component alone remains as the highfrequency component.

The reproduced video signal shown in FIG. 5(d) is obtained as a sum ofthe predictive signal in FIG. 5(b) and the predictive error signal inFIG. 5(c), and contains all frequency components of the input videosignal.

Complete prediction (predictive coefficient: 1) of the high frequencycomponent of the video signal requires a long time until the reproducedhigh frequency component returns to the ordinary condition after randomaccess.

If such a condition is undesirable, reducing the predictive coefficientlower than 1, for instance, to 0.8, can obtain normal picturesrelatively fast although the coding efficiency lowers slightly.

Such processing can be realized by maintaining the high frequencycharacteristic of the LPF 45 in such a level as 20% without completesuppression.

When motion compensation of less than the pixel accuracy is used forinterframe prediction, the motion compensation is applied to the signalof the preceding frame read out of the frame memory 43, and after thatthe signal is fed into the LPF 45.

In this motion compensation resampling processing is conducted and thehigh frequency component is somewhat suppressed owing to thecharacteristic of the filter. In such a case it is unnecessary tocorrect the predictive coefficient due to the characteristic of the LPF45 as described above.

FIG. 6 is a block diagram illustrating an embodiment of the decodingapparatus corresponding to the coding apparatus of FIG. 3.

In FIG. 6, the coded data transmitted from the coding apparatus in FIG.3 via a data input terminal 49 is supplied to an intraframe decoder 51.The decoder 51 decodes the intraframe-coded data to generate areproduced predictive error signal which is then applied to a reversepredictive adder 53.

The adder 53 adds the reproduced predictive error signal to a predictivesignal supplied from a subtractor 55 to generate a reproduced videosignal. The reproduced signal is sent out, via a video signal outputterminal 57, and also is applied to a frame memory 59.

The frame memory 59 supplies an output signal delayed by one frame fromthe input video signal to the LPF 61 and a subtractor 55. The output ofthe LPF 61 is connected to a terminal I of a switch 63. the structureand operation of the switch 63 are the same as those of the codingapparatus shown in FIG. 3. The output signal of the subtractor 55 istherefore supplied, as the same predictive signal as that in the codingapparatus of FIG. 3, to the adder 53.

FIG. 7 is a block diagram showing another embodiment of the predictivecoding apparatus of the present invention. In this embodiment high andlow frequency components of a video signal are respectively encoded, andintraframe prediction of the low frequency component is carried out inall frames.

An input video signal is supplied to a band dividing subtractor 67 andsubsampler 69. In the subsampler 69, filtering as in the case of the LPF45 of FIG. 3 is followed by subsampling of the same frequency band asthat of the LPF 45. In FIG. 4, spatial frequencies, both vertical andhorizontal, are halved.

The output signal of the subsampler 69 is applied to an intraframeencoder 71 and an oversampler 73, The coded data of the low frequencycomponent, which is the output signal of the intraframe encoder 71, istransmitted to a decoder described later, via a data output terminal 75.The encoder 71 processes the subsampled video signal, and the number ofpixels processed by the encoder 71 is one-fourth of the input videosignal.

On the other hand, the oversampler 73 generates pixels deleted by thesubsampler 69 for interpolation and makes the number of pixels the sameas before. The output signal of the oversampler 73 is applied to theband dividing subtractor 67, and subtracted from the input video signal,thus obtaining a video signal having a high frequency component.

The output signal of the band dividing subtractor 67 is led to apredictive subtractor 77, where a predictive signal supplied from memoryframe 79 in subtracted from that output signal. The output of thepredictive subtractor 77, which becomes a predictive error signal, isintraframe-coded by an intraframe encoder 81, the output of which istransmitted to the decoder via the data output terminal 83 and, at thesame time, applied to an intraframe decoder 85. The intraframe encoder81, which encodes the predictive error signal of the high frequencycomponent, processes the same number of pixels as that of the inputvideo signal.

The predictive error signal reproduced by the intraframe decoder 85 isadded to the corresponding predictive signal by a reverse predictiveadder 87, and its output signal is delayed by one frame by the framememory 79, and applied, as a predictive signal, to the predictivesubtractor 77.

FIG. 8 is a block diagram representing an embodiment of the decodingapparatus corresponding to the coding apparatus of FIG. 7. An outputsignal of an intraframe decoder 99 contains a high frequency component,and therefore does not necessitate use of an LPF and a subtractor forpredictive signal generation as in the case of the decoding apparatus ofFIG. 6.

The input data of the low frequency component delivered at a data inputterminal 89 is intraframe-decoded by an intraframe decoder 91, andapplied to an oversampler 93. in the oversampler 93, a signal of thesame number of pixels as that of an original picture is generated andapplied to the adder 95.

On the other hand, the input data of the high frequency componentdelivered from a data input terminal 97 is converted into a reproducedpredictive error signal by an intraframe decoder 99 and applied to areverse predictive adder 101.

The adder 101 adds the reproduced predictive error signal and thepredictive signal supplied from the frame memory 103, thus forming areproduced picture signal of the high frequency component. The formedsignal is applied to an adder 95 and also to the frame memory 103. Theframe memory 103 delays the input signal by one frame and applies thedelayed signal to the reverse predictive adder 101 as a predictivesignal.

The adder 95 adds the signal of the low frequency component fed from theoversampler 93 and that of the high frequency component supplied fromthe adder 101, thereby forming a reproduced video signal which is sentout, via a video signal output terminal 105.

FIG. 9 is a block diagram portraying another embodiment of thepredictive coding apparatus of the present invention. In the embodimentof FIG. 9, an interframe predictive error signal is frequency-divided.

Consequently, after the intraframe decoding of both high and lowfrequency components, a predictive signal is generated by addition ofthe result of the intraframe decoding. The prediction by the predictivesignal carries out motion compensation.

An input video signal, applied via a video signal input terminal 107, issubjected to subtraction of a predictive signal in a predictivesubtractor 109, and its output signal, i.e., a predictive error signal,is applied to a band dividing subtractor 111 and a subsampler 113.

After the same processing in the subsampler 113 as in FIG. 7, the outputsignal is supplied to an intraframe encoder 115, whereas the output ofthe band dividing subtractor 111, which is a high frequency component,is supplied to an intratrame encoder 117. While the intraframe encoder115 is for the subsampled low frequency component, the other intraframeencoder 117 is for the high frequency component.

The output data of both the intraframe encoders are transmitted to adecoding apparatus described later through data output terminals 119 and121, respectively, and also concurrently applied to intraframe decoders123 and 125, respectively.

The output signal of the intraframe decoder 123 is applied to anoversampler 127, and that of the interframe decoder 125 to an adder 129.The output signal of the oversampler 127 is applied to the adder 129 andthe band dividing subtractor 111. The adder 129 adds both signalcomponents applied to it, thereby forming a reproduced predictive errorsignal, which is supplied to a reverse predictive adder 131. Thefunctions of the reverse predictive adder 131, frame memory 133, LPF135, switch 137, and subtractor 139 are the same as in FIG. 3.

On the other hand, a motion compensator 141 detects the motion vector ofevery block by using a reproduced video signal delayed by one frame,which is an output signal of the frame memory 133, and the input videosignal; generates a one-frame delayed signal which has beenmotion-compensated by the vector (although not correctly one-framedelayed because of the motion compensation); and supplies the signal tothe LPF 135 and subtractor 139.

Furthermore data on the motion vector is necessary for the decoder, andtherefore is transmitted from a motion vector output terminal 143 to thedecoder.

FIG. 10 is a block diagram representing the decoding apparatuscorresponding to the coding apparatus of FIG. 9.

Input data entered through data input terminals 145 and 147 isintraframe-decoded by intraframe decoders 149 and 151, respectively. Alow frequency component of the signal delivered from the decoder 149 isapplied to the oversampler 153 in which the number of pixels is made thesame as that of the original picture.

An adder 155 adds a high frequency component delivered from the decoder151 and a low frequency component delivered from the oversampler 153,thereby forming a reproduced predictive error signal.

The functions of a reverse predictive adder 157, frame memory 159, LPF161, switch 163, subtractor 165 are the same as in FIG. 3. The outputsignal of the adder 157 is sent out via a video signal output terminal167 as a reproduced picture.

On the other hand a motion compensator 169 makes motion compensation onthe output signal from the frame memory 159 by means of the values ofmotion vectors supplied from a motion vector data input terminal 171,and feeds the result of compensation to the LPF 161 and subtractor 165.

FIGS. 11A to 11C show frame structure in each of the foregoingembodiments. Each block is a frame, and the broken line separates a highfrequency component (above the line) from a low frequency component(under the line). Letter I denotes a frame coded by independentintraframe coding and letter P a frame coded by interframe predictivecoding. Letter B denotes a frame coded by prediction from the precedingand succeeding frames.

Accordingly, as shown in FIG. 11D, it is also allowed to change theframes of P alone in FIG. 11A to frames of B so as to predict a P-framefrom the preceding P-frame and to predict B-frames from the precedingand succeeding P-frames.

The above embodiments are described so that the signal processing usesframes as a unit of coding and decoding. However, the same descriptioncan be applied to signal processing wherein fields of interlace signalsare used as the unit.

In the embodiments described above in detail, the low frequencycomponents only are subjected to the independent intraframe coding,while the high frequency components are coded by interframe prediction.This type of signal processing makes possible the random access andsearch to the low frequency components as in conventional cases, andimproves coding efficiency of the high frequency components by means ofpredictive processing, thus realizing substantial reduction of thequantity of coded output data as a whole.

In one aspect of random access, although the pictures reproduced ataccess points are composed of low frequency components, the visualcharacteristic of the pictures can be followed without futility. Thatis, because the human sense of sight becomes rather weak directly aftera picture abruptly changes or moves, such delay in the reproduction ofhigh frequency components as in the foregoing embodiments is not soserious of a problem, In addition, smooth search pictures can beobtained by high speed search even though the search is limited to lowfrequency components only.

What is claimed is:
 1. An apparatus for coding a video signal,comprising:means for generating a first interframe or interfieldpredictive signal on the basis of an already coded video signal of frameor field; means for generating a second interframe or interfieldpredictive signal which low frequency components are suppressed bysuppressing low frequency components of the first interframe orinterfield predictive signal; means for generating a predictive errorsignal on the basis of an input video signal and the second interframeor interfield predictive signal; and coding means for intraframe orintrafield coding the predictive error signal.
 2. An apparatus fordecoding a video signal, comprising:means for intraframe or intrafielddecoding a code transmitted from a coding apparatus to obtain a decodedpredictive error signal; means for generating a first interframe orinterfield predictive signal on the basis of an already decoded videosignal of frame or field; means for generating a second interframe orinterfield predictive signal which low frequency components aresuppressed by suppressing low frequency components of the firstinterframe or interfield predictive signal; and reproducing means forreproducing the video signal by adding the decoded predictive errorsignal and the second predictive signal.